In the manufacture of tires, a prime consideration is that each finished tire conform to engineering specifications. Variance from the requisite dimensions of a tire causes the buildup of stress forces within the structure of the tire during use. For example, radial force variation occuring during use of the tire is caused by improper positioning or slippage of the beads relative to each other during first stage construction of the tire carcass, such that the tire formed has an unequal length of tire cord between the beads along its circumference. This may result in premature tire wear as well as unsatisfactory tire performance.
Variance from uniformity and optimum dimensions cn also occur when the beads are placed either too close to or too far from each other. In the former instance, with the beads set too close together, cord length between beads will be insufficient, causing thinning of the innerliner material around the interior surfaces of the tire and excessive or uncontrolled slippage of ply materials around the beads during the curing operation. In the latter instance, with the beads set too far apart, cord length between beads in the resulting tire carcass will be excessive, tending to cause a variety of performance problems including radial force variation and radial runout.
In addition to the foregoing problems which may occur in a single tire when the beads are not properly set with regard to a given tire size, steering and ride problems can develop in a vehicle on which tires of varying dimensions within one general tire size are used, the tires varying because of the possibility of bead distance variation over a production run in previous tire building drums. This variation may be due to either the positioning of the beads before drum expansion, or to the slipping or movement of the beads during or subsequent to expansion. Previous tire building drums have commonly been made adjustable to accomodate a variety of tire sizes such that operator misadjustment or the wear of the adjusting parts can occasion the improper setting of the beads. The surface design of previous drums themselves allows such improper setting of the beads.
There are two basic types of building drums with respect to the positioning and securing of beads to carcass ply material. The first is the non-expanding drum, on which the ply material is bent over the drum edges and the beads are set onto the plies against the sides of the drum. The second type is the expanding drum on which the beads are positioned over the ply material and which is then expanded to permit the plies to contact the beads. This second type includes drums having shallow bead grooves to facilitate positioning of the beads, deep bead grooves to facilitate positioning and securing of the beads, and drums with a formed edge for receiving the beads.
Some early tire building drums had rather steep inclined surfaces at their edges, over which the ply material was wrapped and against which the beads were pressed. Bead slippage and inexactitude of positioning along the inclined surface of the drum could readily occur prior to the wrapping of ply material around the bead. Mere tack adhesion between the bead and green ply material was relied on to secure the bead during the building operation. In addition, the inclined surface failed to present a satisfactory support surface for running on and stitching the body plies, resulting in the folding or crimping and ultimately, the separating of the plies. The securing and accurate positioning of the beads also proved insufficient in the building drums which utilized only shallow bead grooves. Accurate spacing between the beads, and thus required cord length between the beads in the carcass could not be adequately guaranteed because of the possibility of bead travel or slippage during the building operation, and also because of inherent inaccuracies in the positioning of bead setting rings relative to the drum.
Other tire building drums have deeply recessed bead receiving grooves on their surfaces. Body ply materials, which are sheets of uncured elastomeric material, are placed over the surface of the drum, including the recessed areas. Because the body plies are relatively flexible, they tend to fall into the recess and either fold, or crimp. Adequate stitching in those areas is frustrated. The surface of the drum is expanded so as to contact the beads which are positioned about the recessed areas. As the drum expands, the ply material directly over the bead grooves must travel slightly laterally relative to the drum and groove surfaces in order to accommodate the beads in the bead receiving grooves. This lateral travel across the drum and groove surfaces induces further folding, wrinkling, or other deformations which cause inaccuracy in the positioning of the beads and ultimately in the carcass produced.
Still other drums have an initially flat surface on which the body plies are run. The interior portion of the drum is expanded, leaving the edges depressed for receiving the bead. As the interior is expanded, excess body ply fabric may be pulled into the bead receiving area, resulting in folds or crimps before the bead contacts the fabric. Positioning of the bead is also made uncertain in these drums, in which the bead receiving area normally lacks a bead restraint to prevent bead slippage or movement toward the edges of the drum.